Once-through vapor generator

ABSTRACT

A method and apparatus for separately introducing an auxiliary feedfluid into a shell and tube type vapor generator when the flow of main feedfluid is discontinued. The auxiliary feedfluid provides the vapor for preheating the entering main feedfluid when flow of the latter is resumed thereby reducing temperature differentials between the shell and tube sheet metal and the entering main feedfluid so as to eliminate thermal shock.

V Unlted States Patent 1 1111 3,724,532 Sprague [4 Apr. 3, 1973 [54] ONCE-THROUGH VAPOR 3,385,268 5/1968 Sprague ..l22/32 GENERATOR 2,862,479 12/1958 Blaser eta] ..l22/34 [75] Inventor: Theodore S. Sprague, Hudson, Ohio primary Examiner Manuel Amonakas [73] Assignee: The Babcock 8: Wilcox Company, Magun'e New York, N.Y. 1

22 Filed: Oct. 27, 1971 [57] ABSTRACT [21] Appl. No.: 192,963

y 7 A method and apparatus for-separately mtroducmg an Related US. Application Data auxiliary feedfluid into a shell and tube type vapor [62] Division of Ser No 483 March 2 1970 Pat No generator when the flow of main feedfluid is discontinued. The auxiliary feedfluid provides the vapor for preheating the entering main feedfluid when flow of 1 the latter is resumed thereby reducing temperature 33? differentials between the shell and tube sheet metal [58] Field of Search ..122/32, 34; 165/1, 140, 158, 3: f g i mam feedflmd 5 to 9 v 165/108 erma s oc.

[56] R t C" d 7 Claims, 3 Drawing Figures e erences e UNITEDSTATES PATENTS 3,576,178 4/l97l Zmola .Q ..l22/32 i 33 I 33 F i F r 34 2 f 25 1 a i 35 2 24 W 26 11 I f 38 j Hi 38 r I 30 :1; 30 o 1 3O 1 3 LaoA 30A Q 31 3 PATENTED N I975 SHEET 1 [IF 2 FIG ONCE-THROUGH VAPOR GENERATOR CROSS-REFERENCE TO RELATED APPLICATION This application is a division of our application Ser. No. 15,483 filed on Mar. 2, 1970 and now US. Pat. No. 3,635,287.

BACKGROUND OF THE INVENTION present invention wherein a heating fluid is directed through the tubes, and the feed-fluid is discharged into the shell and flows downwardly through an annular downcomer and then upwardly through a tube bank and is heated and vaporized as it passes over and along the tubes. A portion of this partially-heated fluid is withdrawn from the tube bank to mix with and preheat the feedfluid entering the vapor generator shell, thereby eliminating thermal shock of the thick shell and lower tube sheet metal due to the temperature differential between the metal and the entering feedwater. In order to avert costly repairs and to prolong the life of the vapor generator, stringent metal temperature differential limits have been established for the startup and shutdown of the plant. These operational limitations, however, have the disadvantage of extending the outagetime thereby imposing an economic loss due to reduction of plant availability. A particularly disadvantageous situation arises when, due toan emergency condition, the flow of feedfluid may be interrupted and the vapor generator heating fluid is evaporated and its metal temperature may approach the temperature of the entering heating fluid. Under these circumstances. even though the emergency condition may be quickly corrected, there will be no heated fluid available to preheat the feedfluid entering the generator shell and the plant may be forced into a protracted shutdown while the vapor generator metal temperatures decrease to a point wherein the addition of feedfluid without the benefit of preheating will not cause temperature differentials in excess of the prescribed limits.

SUMMARY OF THE INVENTION In accordance with the present invention an improvement is made on vapor generators of the'type disclosed in U. S. Pat. Nos. 3,385,268 and 3,447,509 by providing an arrangement whereby the introduction of main feedfluid into the vapor generator can be resumed without delay following a stoppage in the flow of feedfluid during the operation of the vapor generator. The arrangement includes a plurality of nozzles penetrating the upper end of the shroud to discharge a controlled quantity of auxiliary feedfluid directly into the inner passage of the vapor generator. A typical auxiliary feedfluid system has a maximum flow capacity of approximately 3.5 percent of the full main feedfluid flow. The shroud and heat exchange tubes are relatively thinwalled and able to withstand. the temperature differentials resulting from the introduction of non-preheated auxiliary feedfluid. Following the loss of main feedfluid flow, auxiliary feedfluid is injected directly into the inner passage and vaporizes as it comes into contact with the hot tubes, resulting in a vapor pressure buildup within the generator. Prompt resumption of main feedfluid flow to the vapor generator is made possible through the use of auxiliary feedfluid vapor to preheat the entering main feedfluid. As soon as normal vapor generation is established, the flow of auxiliary feedfluid can be discontinued. 1

, An alternative use can be made of the auxiliary feedfluid system to remove decay heat from the vapor generator and its-associated heat source, e.g., a nuclear reactor, during a plant shut-down. A predetermined rate of cooling can be maintained by regulating the quantity of auxiliary feedfluid introduced into the vapor generator and discharging the resulting vapor directly to the condenser.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional elevation view of a once-through vapor generator embodying the invention;

FIG. 2 is a transverse section taken along line 2-2 in FIG. 1.

FIG. 3 is a transverse section taken along line 3-3 in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT tube sheets 14 and 15 respectively. The upper tube sheet 14' is integrally attached to vessel 11 and upper head member 12 and forms in combination with the upper head member a fluid inlet chamber 16. The lower tube sheet 15 is integrally attached to vessel 11 and lower head member 13 and forms in combination with the lower head member a fluid outlet chamber 17.

A multiplicity of straight tubes 18 arranged to form a tube bank extend vertically between the upper and lower tube sheets 14 and 15 and penetrate through both tube sheets to interconnect the fluid inlet chamber 16 with the fluid outlet chamber 17. A cylindrically shaped lower shroud member 19 surrounds the tubes 18 and extends upwardly from the upper face of the lower tube sheet 15 and terminating at a plane intermediate the height of vessel 11. This lower shroud defines the lower portion of an inner passage or steam generating riser chamber 20 which contains the lower portion of tubes 18 and cooperates with the vessel 1 1 to form the lower portion of a circumscribing annular shaped outer passage or inlet compartment 21. Openings 22 circumferentially spaced about the lower portion i of shroud 19 provide flow communication between the inlet compartment 21 and the riser chamber 20. An adjustable circular segmental plate orifice 23 projects outwardly from the shroud 19 at approximately the level of the top edge of openings 22.

A cylindrically shaped upper shroud member 24 extends upwardly from a plane closely spaced above the upper edge of lower shroud 19 to aplane located below the upper tube sheet 14. This upper shroud 24 forms the upper portion of an inner passage or steam generating and superheating chamber 25 and being an extension of chamber 20, contains the upper section of tubes 18. The shroud 24 in cooperation with the vessel 11 forms the upper portion of an annular shaped outer passage or outlet compartment 26. The lower end compartment 26 is sealed closed by an annular plate 27 welded about its outer edge to the vessel 11 and around its inner edge to the shroud 24. The open space 28 between the top edge of shroud 19 and the bottom plate 27 of shroud 24 is in flow communication with the inlet compartment 21. A number of tube supports 29 are spaced along the length of the bank of tubes 18 within the chambers 20 and 25.

At the upper end of the inlet compartment 21, a plurality of main feedfluid nozzles 30 extend through the wall of vessel 11 with their respective outlet ends discharging into the inlet compartment 21 near or at the same level as the open space 28 and as shown by the spray pattern at 30A. Connecting pipes 31 join nozzles 30 to a ring shaped main feedfluid heater 32 which encircles the vessel 1 1 below the nozzles 30.

Near the upper end of the outlet compartment 26, a plurality of auxiliary feedfluid nozzles 33 extend through the wall of vessel 11 and the upper shroud 24 with their respective outlet ends discharging into the steam generating and superheating chamber 25 as shown by the spray pattern at 33A. Connecting pipes 34 join nozzles 33 to a ring shaped auxiliary feedfluid header 35 which encircles the vessel 11 below the nozzles 33.

The upper head member 12 is provided with an inlet connection 36 for admitting heating fluid to chamber 16 while lower head member 13 is provided with an outlet connection 37 for discharging the heating fluid from chamber 17. The vessel 11 includes outlet connections 38 for delivering the superheater vapor to the point of use, and manway 39 and inspection ports 40 and 41 which provide physical and visual access to the interior of the vessel. Also included are fluid level sensing connections 42, vent connection 43 and drain connections 44. The upper and lower heads 12 and 13 are provided with manways 45 and 46 and inspection ports 47 and 48 respectively and lower head 13 also includes a drain connection 49.

FIG. 2 illustrates a transverse'section of the oncethrough vapor generating and superheating unit taken at section 2-2 of FIG. 1, i.e., at the auxiliary feed inlet to the unit. The auxiliary feedfluid nozzles 33 are shown spaced circumferentially about the vessel 1 l and extending extending through the vessel wall, across the outlet compartment 26 and through the upper shroud member 24 to discharge directly into the upper portion of the inner passage of the generating and superheating chamber 25 as shown at 33A. An auxiliary feedfluid header 35, made up of two arcuate sections joined by a flanged connection 50, supplies fluid through the connecting pipes 34 to the nozzles 33 for discharge over the outside of tubes 18.

FIG. 3 illustrates a transverse section of the oncethrough vapor generating and superheating unit 10 taken at section 3-3 of FIG. 1, i.e., at the main feedfluid inlet to the unit and including the multiple main feedfluid nozzles 30, only five of which are actually shown, spaced circumferentially about the vessel 11 and extending through the vessel wall to discharge directly downward into the inlet compartment 21. A main feedfluid header 32, made up of two separate arcuate sections, supplies fluid through the connecting pipes 31 to the nozzles for discharge into compartment 21. The lower shroud member 19 defines the outer periphery of the lower portion of the inner passage or riser chamber 20 which houses the lower length section of tubes 18.

During normal operation of the vapor generator, primary coolant received from a pressurized water reactor or a similar source, not shown, is supplied to the upper chamber 16 through the inlet connection 36. The primary coolant gives up heat to a secondary fluid during passage through the tubes 18 of vapor generator 10 and thus will hereinafter be referred to as the heating fluid. From chamber 16, the heating fluid flows downwardly through the tubes 18 into the lower chamber 17 and is discharged from the vapor generator through the outlet connection 37. The feedfluid supplied to the header 32 from whence it is discharged through the nozzles 30 into the upper end of the inlet compartment 21 of the vapor generator. The feedfluid flows downwardly through the inlet compartment 21 and past the adjustable orifice 23 and through the shroud openings 22 into the riser chamber 20. The main feedfluid enters the riser chamber 20 at substantially saturation temperature and vapor generation commences immediately. It flows upwardly about the tubes in counterflow and indirect heat transfer relationship with the heating fluid flowing within the tubes 18.

As the main feedfluid flows upwardly through the riser chamber 20, vapor is generated ranging from zero quality at the lower tube sheet 15 to substantially percent quality adjacent the upper end of the lower shroud 19. A portion of the main feedfluid in the form of vapor at substantially 100 percent quality is withdrawn from the top of shroud 19 and passed through the open space 28 to mix with and heat the main feedfluid being discharged from the nozzles 30. As this vapor mixes with the incoming feedfluid, it condenses resulting in a slight reduction in pressure which provides an aspirating effect causing the withdrawal of vapor from within the chamber 20 into the inlet compartment 21. The withdrawn vapor gives up its latent heat of vaporization to the incoming feedfluid with the mixture being heated substantially to saturation temperature. That portion of vapor which has not been withdrawn is passed upwardly through the superheating chamber 25 and is super-heated before it reverses direction about the upper shroud 24. It then flows downwardly through the outlet compartment 26 between the upper shroud and the shell and finally exits from the unit through the vapor outlet connections.

In accordance with the present invention, whenever there is a complete stoppage of the main feedfluid supply to the vapor generator, the nuclear reactor is automatically shutdown while the primary coolant or heating fluid continues to pass through the reactor and through the vapor generator at a selected rate of flow. The auxiliary feed-fluid system becomes activated substantially simultaneously with the loss of the main feedfluid supply. Auxiliary fluid is supplied to the header 35 and injected through the nozzles 33 directly into the upper portion of the inner passage or superheating chamber 25 some of which vaporizes as it comes into contact with the heated tubes 18 resulting in a vapor pressure build-up within the generator. Auxiliary feedfluid continues to flow in at a selected rate of flow so as to maintain a preset minimum water level. Decay heat from the reactor is removed by continuing the indirect. heat exchange between the primary coolant and the auxiliary feedfluid and discharging the resulting steam directly to the condenser.

In the event that a hot restart is contemplated, introduction of main feedfluid to the generator may be resumed as soon as the supplyv becomes available. The possibility of thermally shocking the hot metal of the vessel 11 and the tube sheet 15, due to temperature differentials created by the relatively cool incoming main feedfluid, is eliminated by withdrawing the vapor portion of the auxiliary fluid through the open space 28 and mixing it directly with the main feedfluid being discharged by the nozzles 30 thereby preheating the latter fluid before any substantial contact is made with the heated generator metal. The flow of auxiliary feedfluid is continued until normal main feedfluid vapor generation is resumed at which time the flow of auxiliary fluid is discontinued.

In the event that the plant is scheduled for a shutdown, the rate of cooling forthe nuclear reactor and generating equipment may be predetermined by regulating the quantity of auxiliary feedfluid flowing intothe vapor generator.

In a typical nuclear powered steam generator, the

auxiliary feedwater system has a flow range from to approximately 3.5 percent of full load main feedwater flow and is capable of removing up to percent of reactor power, assuming a feedwater inlet temperature of 90" F and generation of saturatedsteam at full load steam pressure conditions.

What is claimed is: 1. The method of operating a tube and shell heat exchanger comprising:

passing a heating fluid through the tubes, introducing a first feedfluid in a space between the tubes and the shell and separated from contact with said tubes, directing said first feedfluid over the tubes in indirect heat absorbing relation with the heating fluid,

withdrawing a portion of the partially-heated first feedfluid,

mixing said portion of partially-heated fluid with the first feedfluid entering the shell,

discharging the remaining heated first feedfluid, and

separately introducing a second feedfluid directly over the tubes when flow of said first fluid is discontinued. 2. The method of operating a tube and shell heat exchanger according to claim 1 which includes:

initiating the introduction of second feedfluid substantially simultaneous with the disconuance of flow of said first feedfluid. i 3. The method of operating a tube and shell heat exchanger according to claim 1 which includes:

heating the second feedfluid to a selected temperature before restunption of flow of said first feedfluid. 4. The method of operating a tube and shell heat exchanger according to claim 3 which includes: mixing the heated second feedfluid with the first feedfluid entering the shell upon resumption of flow of the last named fluid. 5. The method of operating a tube and shell heat exchanger according to claim 4 which includes:

discontinuing the flow of second feedfluid when the first feedfluid has attained a selected temperature. 6. The method of operating a tube and shell heat exchanger according toclaim 1 which includes:

limiting the maximum'flow quantity of the second feedfluid to a small percentage of the maximum flow quantity of said first feedfluid. 7. The method of operating a tube and shell heat exchanger according to claim 1 which includes:

regulating the flow rate of the second feedfluid to provide a predetermined rate of heat absorption. 

1. The method of operating a tube and shell heat exchanger comprising: passiNg a heating fluid through the tubes, introducing a first feedfluid in a space between the tubes and the shell and separated from contact with said tubes, directing said first feedfluid over the tubes in indirect heat absorbing relation with the heating fluid, withdrawing a portion of the partially-heated first feedfluid, mixing said portion of partially-heated fluid with the first feedfluid entering the shell, discharging the remaining heated first feedfluid, and separately introducing a second feedfluid directly over the tubes when flow of said first fluid is discontinued.
 2. The method of operating a tube and shell heat exchanger according to claim 1 which includes: initiating the introduction of second feedfluid substantially simultaneous with the disconuance of flow of said first feedfluid.
 3. The method of operating a tube and shell heat exchanger according to claim 1 which includes: heating the second feedfluid to a selected temperature before resumption of flow of said first feedfluid.
 4. The method of operating a tube and shell heat exchanger according to claim 3 which includes: mixing the heated second feedfluid with the first feedfluid entering the shell upon resumption of flow of the last named fluid.
 5. The method of operating a tube and shell heat exchanger according to claim 4 which includes: discontinuing the flow of second feedfluid when the first feedfluid has attained a selected temperature.
 6. The method of operating a tube and shell heat exchanger according to claim 1 which includes: limiting the maximum flow quantity of the second feedfluid to a small percentage of the maximum flow quantity of said first feedfluid.
 7. The method of operating a tube and shell heat exchanger according to claim 1 which includes: regulating the flow rate of the second feedfluid to provide a predetermined rate of heat absorption. 